1. Field of the Invention
The present invention relates to a photorefractive material and composition which have a photorefractive effect, and, more particularly, to a holographic recording medium which is used in a volume holographic memory or the like.
2. Description of the Related Art
Conventionally, a holographic memory system is known as a digital recording system using the principle of holography. The holographic memory system records digital data on a memory medium made of a photorefractive crystal such as lithium niobate (LiNbO3) or the like, and reproduces the data from the same. The photorefractive effect is a phenomenon in such that electric charges generated by photo-excitation move within a crystal thereby to form a spatial electric field distribution, which combines with a primary electro-optical effect i.e., Pockels effect to change a refractive index distribution in the crystal. In a ferroelectric crystal or the like exhibiting the photo-refractive effect, its change of the refractive index is responsive even to a fine optical input pattern of 1,000 lines or more per one millimeter. This effective action is generated at a response speed on the order of microseconds to seconds in real time, though the response speed varies depending on kinds of materials. Therefore, a variety of applications for such crystals has been studied as a real time hologram medium which does not require any developing the medium.
The holographic memory system is capable of recording and reproducing data on a two-dimensional plane page unit, and also performing a multiple recording with use of a plurality of the page units. The volume holographic memory is designed to enable three-dimensional recording with a crystal medium being of a three-dimensional shape such as a rectangular parallelepiped or the like.
As shown in FIG. 1, there are two recording modes: a single color (1-color) hologram system and a two-color hologram system. The single color hologram system performs recording and reproduction using interference of information-carrying coherent light with one wavelength xcex1 and uses LiNbO3 crystal as a medium to which, for example, iron (Fe) is added or doped. In the portion of the crystal which has bright interference fringes, light is absorbed and then electrons excited to the conduction band CB from the level xe2x80x9cAxe2x80x9d of Fe2+ (the center of light absorption), are diffused into a dark portion and eventually trapped at the level xe2x80x9cCxe2x80x9d of Fe3+ (the deep trap level or storage center) as shown in FIG. 1. The electrooptic effect that is induced by the spatial distribution of the densities of electrons produced that way produces a spatial refractive index distribution corresponding to the interference fringes in the crystal so that information can be stored.
The single color hologram system has a problem that a medium has a sensitivity to light of one wavelength that is used at the time of recording and reproduction. In a single color hologram, recorded information is electrons trapped at the trap level (storage center xe2x80x9cCxe2x80x9d) which is produced by Fe, so that what is called reproduction deterioration occurs. That is, every time reproduction is performed, electrons are gradually excited to the conduction band CB from the trap level, thereby erasing the stored information. According to the conventional holographic memory, at the time signals are read from a hologram recorded there, reproduction light gradually erases the hologram.
The two-color hologram system can suppress the reproduction deterioration. As shown in FIG. 1, the characteristic of the two-color hologram system lies in that at the time of recording, reference light and signal light (wavelength xcex1) of recording light that forms a hologram, and another light called xe2x80x9cgate lightxe2x80x9d (wavelength xcex2) are irradiated simultaneously to record the hologram. The gate light provides the crystal with the recording sensitivity at the wavelength (xcex1) of the recording light only during the irradiation of the gate light. This property temporarily forms carriers at a relatively shallow energy level called xe2x80x9cintermediate excitation level Bxe2x80x9d in the crystal only in the irradiated portion and only for the irradiation time. The carriers of the intermediate excitation level xe2x80x9cBxe2x80x9d is excited to the conduction band CB by the recording light (a spatial contrast pattern corresponding to the interference fringes produced by the reference light and signal light) and are finally stored at the deep trap level xe2x80x9cCxe2x80x9d in the form of the carrier density distribution corresponding to the interference fringes. This completes recording.
Recording of a two-color hologram with LiNbO3 was announced by von der Linde, et al. in 1974 (D. von der Linde, A. M. Glass and K. F. Rodgers: xe2x80x9cMultiphoton photorefractive processes for optical storage in LiNbO3xe2x80x9d, Appl. Phys. Lett., Vol.25, pp. 155-157 (1974). Because the materials available then had a short carrier life (of a nano-second order) at the intermediate excitation level, recording was possible only with a pulse laser which has large peak power. Crystals available today that are acquired by performing reduction on LiNbO3 doped with Pr (praseodymium) (H. Guenther, G. Wittmann, and R. M. Macfarlene (IBM), R. R. Neurgaonkar (Rockwell): xe2x80x9cIntensity dependence and white-light gating of two-color photorefractive gratings in LiNbO3xe2x80x9d, Opt. Lett. Vol. 22, pp. 1305-1307 (1997), or LiNbO3 which contains no additive or is doped with Fe and whose composition ratio is close to a stoichiometric composition ratio (the latter abbreviated to SLN) (H. Guenther, R. M. Macfarlane, Y. Furukawa, K. Kitamura; xe2x80x9cTwo-color holography in reduced near- stoichiometric lithium niobatexe2x80x9d, Appl. Opt. Vol. 37,pp. 7611-7623 (1998) can increase the carrier life to micro seconds to several seconds in the intermediate excitation level (metastable level) and can permit recording by using a laser of continuous wave oscillation that has relative small power.
The following discusses the mechanism of exciting carriers that dedicate to a two-color hologram in a LiNbO3 single crystal undergone reduction (bipolaron/polaron mechanism). When a LiNbO3 single crystal is subjected to a heat treatment under a proper reduction atmosphere, a bipolaron is formed. The bipolaron is the state where a single electron is trapped by each of adjoining NbLi (Nb of the Li site) and NbNb (Nb of the Nb site) and those electrons form a pair. The bipolaron forms a wide absorption band whose center lies around 2.5 eV. When electrons are excited from the bipolaron absorption band by the irradiation of the gate light that has a wavelength of 400 nm to 500 nm, the bipolaron is photodissociated and a small polaron state appears. The small polaron is the state where an electron is trapped by NbLi (Nb of the Li site) and this state corresponds to an electron being present at the intermediate excitation level.
Two-color holographic recording media which use this bipolaron mechanism have the following shortcomings.
(1) Reduction is required. That is, this type of recording medium can hardly be used as a recording material because of a low sensitivity in the as-grown state or after a heat treatment performed in the air.
(2) Excessive reduction increases the dark conductivity of a crystal, thus shortening the storage time, which raises practical problems.
(3) As the degree of reduction performed significantly changes the characteristic, it is difficult to control the characteristic.
(4) With regard to materials doped with Fe or the like, reduction causes most of Fe3+ to be reduced to Fe2+. This decreases the density of Fe3+ necessary as a trap and Fe2+ enhances absorption, thus raising practical problems. There is a demand for the development of practical materials that do not require reduction.
Thus, it is an object of the present invention to provide a photorefractive material which provides high photosensitivity without reduction.
A photorefractive material according to the present invention characteristically comprises a single crystal of niobate or tantalate doped with terbium (Tb).
In the photorefractive material, the single crystal may be a single crystal of lithium niobate (LiNbO3) whose molar fraction of [Li2O]/([Li2O]+[Nb2O5]) lies in a range of 0.482 to 0.505 preferably 0.490 to 0.505.
In the photorefractive material, the single crystal may be a single crystal of lithium tantalate (LiTaO3) whose molar fraction of [Li2O]/([Li2O]+[Ta2O5]) lies in a range of 0.482 to 0.505 preferably 0.490 to 0.505.
In the above photorefractive material, the amount of terbium added may range from 10 weight ppm to 1000 weight ppm.
In an aspect of the present invention, said single crystal contains Fe, Mn, Cr or Ni in addition to terbium.
In another aspect of the present invention, an amount of Fe, Mn, Cr or Ni added ranges from 1 weight ppm to 500 weight ppm.
In a further aspect of the present invention, a two-color holographic recording medium comprising a photorefractive material above mentioned is formed into a predetermined shape such as cube, cylinder, sphere, disk, rectangular parallelepiped, polyhedron or the like.